1. Field of the Invention
This invention relates to an activation rupture disc unit capable of withstanding a substantially higher back pressure than its burst activation pressure. A major problem in oil exploration and recovery is the failure of pressure activation devices. Although the rupture disc unit has particular utility for activating any one of a number of oil field downhole completion and production devices, it can be used for other applications requiring withstand of a high differential back pressure including deep sea, aerospace, transformers, and high pressure vessels.
The activation disc is preferably capable of withstanding at least about 20,000 psig differential back pressure, or higher, e.g., 35,000 psig up to as much as about 50,000 psig, while being rupturable at an activation pressure of the order of 1,000 to 8,000 psig. The burst pressure for a ¼ in. diameter disc will be about 1,000 psig, and lower than that pressure for larger size discs up to 90% of the operating ratio. In addition, the rupture disc of the unit desirably will not unduly fatigue when subjected to at least about 100 cycles of full back pressure to 90% of the burst pressure, and must reliably operate within a temperature range as low as 40° F. to as much as 350° F., that typically may be encountered downhole in an oil well.
2. Description of the Prior Art
Rupture discs have previously been employed in a variety of downhole oil well applications. In U.S. Pat. No. 5,996,696, apparatus is provided for testing the integrity of oil delivery tubing with an oil well casing. A tubular holder connected to the lower of a series of end-to-end pipe string sections is provided with a rupture disc normally closing the through passage in the holder. The pipe string, with the holder in place, is subjected to liquid pressure sufficient to test the integrity of the connection between adjacent pairs of tubing sections. Once it has been determined there are no leaks in any of the pipe string section connections, the tubing string is over pressured to an extent to rupture the disc, thereby opening the holder and providing a through passage communicating with the pipe string. In this well test rupture disc application, the pressure on opposite sides of the rupture disc never exceeds the ability of the disc to withstand the differential back pressure.
In another downhole rupture disc application, a rupture disc is mounted in the sidewall of a tubular holder connected to the lowermost section of a pipe string, immediately above a sucker rod or other type of pump. When maintenance or replacement of the pump is required, which can vary in frequency from weeks to months depending upon downhole conditions, the tubing string can be over pressured from a surface pressure source to an extent to rupture the disc, thereby allowing liquid in the pipe string to drain from the string as the sections are sequentially pulled to the surface and disconnected one at a time. In this manner, spillage of oil from each section as it is withdrawn from the well and disconnected from the pipe section there below is avoided. Here again, the pressure differential on the dump valve disc is not so great as to cause premature rupture of the disc.
There has been a longstanding need in the art, though, for an activation rupture disc unit that is capable of withstanding very high variable differential back pressures, but at the same time may reliably be selectively opened under a significantly lower activation pressure.